In recent times, greater emphasis has been placed on national home security and detecting threats to populations. In particular, detecting or sensing the presence of undesired chemicals or biological material in the environment has become a priority, and a variety of detection devices have been developed in response thereto. One possible implementation of a chemical sensor is a sensor using a multi-mode optical fiber having a glass core and a cladding surrounding the core. The cladding, or a coating on the cladding, has optical properties which are altered in the presence of a pre-determined material to be detected. As light is transmitted through the core of the optical fiber, the optical properties of the light vary with respect to changes in optical properties of the cladding or coating interacting with the material to be detected. Some advantages of multi-mode fiber are that the core has a diameter with sufficient structural integrity and can be coated with a chemically sensitive polymer. Some of the core-guided light can interact with this polymer.
Resonators have been implemented in chemical sensors to circulate light around an optical fiber loop for multiple passes. A periodic series of resonance lineshapes is produced, each having a peak centered about a resonance frequency under normal conditions, and the resonance lineshape has a finesse associated therewith. The frequency-periodicity of frequency separation between resonance frequencies of the same mode is the free spectral range of the resonator. As used herein, the term “finesse” refers to a relationship (e.g., sharpness) based on a ratio of the free-spectral range to the linewidth of an individual resonance lineshape. The linewidth of the resonance lineshape is a frequency width at half of the maximum peak value of the resonance lineshape. The finesse additionally relates to the number of times the light recirculates within the optical loop with reproducibility, and thus is inherently related to the round-trip loss of the resonator. Higher losses generally result in lower finesses. It is generally difficult to couple light into a multi-mode optical fiber and maintain the light in a single spatial mode that reproduces itself for multiple circulations through the resonator. For example, perturbations (e.g., imperfections, geometrical distortions, etc.) along the length of the optical fiber typically decrease the round-trip reproducibility of the single fiber spatial mode within a multi-mode fiber, and thus decrease the finesse. Other spatial mode resonances can also be excited which typically cause errors in the intended measurement. In the latter case, a complex structure of resonances, which may be based on a single stable resonance, may be observed that create instabilities and errors in the measurement. Each spatial mode may be associated with two polarization modes, which doubles the number of resonances in the spectrum.
A single mode optical fiber may be used to significantly improve the resonance characteristics of the resonator by assuring that a single spatial mode of the fiber supports the resonance mode of the resonator. For example, this single spatial mode is the sole resonating mode provided that one polarization state is resonating within the resonator. Instabilities created by power sharing between several spatial modes of the fiber and errors resulting from the presence of several resonator modes are thus substantially eliminated. Measurements of the finesse, the linewidth of the resonance, and the free spectral range are typically unique since these relate to the loss and pathlength for light traveling within a single spatial mode of the fiber and for a single resonance lineshape. To make a chemically sensitive fiber, the light should interact with the polymer. Placing a permeable, chemically sensitive polymer cladding directly on the core of a typical single mode fiber is generally impractical because the core is too small (e.g., about 5-10 μm). Such a small glass diameter typically lacks the mechanical strength normally associated with larger chemically sensitive multi-mode optical fibers. Applying a relatively pliable cladding or coating around the core of a single mode optical fiber is also difficult in practice. Adding an intermediate glass cladding between the core and a polymer coating would tend to interfere with sensing.
Accordingly, it is desirable to provide a resonator-based sensor for detecting the presence of chemical and/or biological agents having a high finesse and a stable resonance structure corresponding with the light traveling within a single spatial mode of the fiber. In addition, it is desirable to provide a resonator-based sensor for detecting the presence of chemical and/or biological agents using a single-mode fiber. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.